On New Zealand’s North Island, rolling hills and river valleys separate bays and beaches and are home to a unique collection of plants and birds. But since the introduction of nonnative bird predators—particularly cats, rats, and stoats—several species of birds have become locally extinct. And as biologists Sandra H. Anderson and Dave Kelly recently reported, this decline in bird populations has triggered a cascade of ecological change that is now reducing the density of bird-pollinated plants.

In their latest study, published in the journal Science, Anderson and Kelly, who are based at the universities of Auckland and Canterbury, respectively, linked the 19th-century extirpation from the upper North Island of two bird species—the bellbird (Anthornis melanura) and stitchbird (Notiomystis cincta)—with reductions in pollination of a shrub known as Rhabdothamnus solandri. A third pollinating bird—the tui (Prosthemadera novaeseelandiae)—survives in reduced numbers on the upper North Island but rarely visits flowers.

Anderson and Kelly’s work has described, for the first time, a terrestrial trophic cascade in the North Island that has fundamentally altered local ecology. Trophic cascades often begin with the introduction of a predator, which causes shifts in ecosystem structure and function, typically with consequent losses in biodiversity.

The Decline of Birds and Pollination on the North Island

Anderson and Kelly’s investigation of pollination of Rhabdothamnus followed from earlier research by biologist Jenny Ladley, a co-author on the Rhabdothamnus work, who in the early 1990s showed that mistletoes (Peraxilla species) were suffering from low natural pollination by birds at many mainland (North and South Island) sites. “So, we started testing other New Zealand plants with bird-adapted flowers,” Kelly said.

Because Rhabdothamnus grows both on the North Island, where its pollination is limited, and on offshore islands, where its pollination thrives, it represented an ideal subject for investigating the consequences of bird declines. That birds might be a key factor was made apparent by Anderson’s observations of bird-pollinated plants. As she explained, “From fieldwork on offshore islands, I knew there was a whole suite of native flowering plants receiving bird visitation. But on the adjacent mainland, these same plants were rarely visited by birds.”

When Anderson and Kelly began their research, very little was known about Rhabdothamnus. In fact, it was not even clear whether the shrub was pollinated by birds or by insects. “As far back as 1902, botanist Donald Petrie was puzzling over what pollinated Rhabdothamnus,” Anderson noted.

After a meticulous exploration of the effects on pollination of bird exclusion from flowers, the team successfully identified the shrub’s primary pollinators—the bellbird, stitchbird, and tui. According to Kelly, “The two key birds (bellbirds and stitchbirds) that were lost off the North Island, and that were vital for Rhabdothamnus pollination, were lost very rapidly about 1870.”

The researchers worked on the offshore islands of Little Barrier, Tiritiri Matangi, and Lady Alice, which still have bellbirds and (for the first two islands) stitchbirds, and are protected as nature reserves and provide rich bird habitat. The tui, however, while found on these islands, also inhabits the mainland. The disparity in pollinator presence between the mainland and the offshore islands, compared with patterns of pollination failure and success, suggested that the shrub’s limited pollination on the North Island is due to the lack of bird pollinators there.

The team also investigated whether other birds or insects were acting as replacements to maintain the plant’s reproductive success, but the only other visitors identified in the study were recently self-introduced birds known as silvereyes, which usually rob the nectar without pollinating. However, as Anderson mentioned, when nectar accumulates sufficiently in the calyx, these birds will occasionally pollinate the flowers.

Preventing the Cascade into Extinction

The decline of birds is a worldwide phenomenon on islands where bird predators, such as cats and rats, have been introduced. The Hawaiian islands, for example, have suffered devastating losses of birds as a result of predator introductions.

In many instances, declines in birds, and the ensuing cascades of change, tend to be inconspicuous, at least at first. But eventually ecosystem services become altered dramatically, to the point that other native species may undergo drastic declines. As an example, Anderson cited the extinction of the New Zealand mistletoe Trilepidia adamsii, which has been linked to the loss of birds throughout the plant’s range in the North Island.

To prevent further losses of birds and plants in New Zealand, researchers and local officials are relying increasingly on conservation. In the case of the North Island, as Kelly said, “We have to get the birds back, and luckily we can do this by using a predator-proof fence to create a sanctuary on the mainland and by eradicating pests from additional islands to rehabilitate these areas and restore birds.”

The fences to which Kelly refers protect large areas of habitat and are known as mainland islands (or mainland sanctuaries). “There are more than 30 of them on the main islands of New Zealand now,” he said.

He explained too that local conservation efforts have been very popular with the public. “One of the island bird sanctuaries we studied, Tiritiri Matangi, was previously farmed, and its current state is the result of a 30-year, citizen-driven restoration project. It is an open sanctuary and has thousands of visitors every year.”

Anderson and Kelly are hopeful that a spillover of birds from both island and mainland sanctuaries will benefit the pollination and seed dispersal of bird-dependent plants outside the sanctuaries. In the meantime, the team plans to obtain a more extensive knowledge of local plant pollination and dispersal.

“We would like to know if fruit dispersal is failing, since this also suffers from reduced bird densities, especially for large-fruited plants,” Kelly said. “The key process being disrupted is long-distance dispersal. We hope to study this through modeling, coupled with radiotracking data on fruit-eating birds.” Dispersal, however, is much more difficult to study than pollination, “which is why nobody has really completely worked it out yet, anywhere,” he added.

Anderson explained that another area of work will entail the assessment of long-term effects of pollination reduction on other flowering plants that may be in the same predicament as Rhabdothamnus. “The idea is to be able to predict which interactions are most vulnerable before it is too late,” she said. “We also plan to look at other mutualistic interactions like seed dispersal, particularly since some of the flowering plants that are bird-pollinated also have fleshy fruited seeds and rely on the same birds for pollination and dispersal.”

About Science Up Front

A regular Britannica Blog feature written by the encyclopedia’s own Kara Rogers, Science Up Front goes behind the headlines to bring researchers’ stories of discovery centerstage. Begun in 2009 to highlight the ingenious work of pioneering scientists and to bring greater accuracy to science reporting, Rogers goes straight to the source, exploring the latest advances in science, from medicine to nanotechnology to conservation, through first-hand interviews with researchers. The series covers all things science, so check back regularly to see who’s up on Science Up Front.